1. Field of the Invention
The invention relates to a method for imaging a pattern arranged in an object plane of a projection lens onto an image plane of the projection lens, a projection lens for carrying out the method, and a method for fabricating such a projection lens.
2. Description of the Related Art
Illumination methods and projection lenses of the types are employed on projection illumination systems, such as wafer scanners or wafer steppers, that are used for fabricating semiconductor devices or other types of microelectronic devices and serve to project reduced images of patterns present on photomasks or reticles, which shall hereinafter be generically referred to as “masks” or “reticles,” at ultrahigh resolution onto an object that has been coated with a layer of photosensitive material.
Improving the spatial resolutions of the projected images of masks having increasingly finer patterns will necessitate both increasing the numerical aperture (NA) of the image end of the projection lens involved and employing light having shorter wavelengths, preferably ultraviolet light having wavelengths less than about 260 nm.
There are few materials, in particular, synthetic quartz glass and crystalline fluorides, such as calcium fluoride, that are sufficiently transparent at such short wavelengths available for fabricating the optical components involved. However, the materials suffer from photoelastic effects, i.e., may affect the mutually orthogonally polarized components of the field vectors of transmitted light differently due to stress birefringence.
Since the Abbé numbers of those materials that are available all lie rather close to one another, it is difficult to configure systems consisting entirely of refractive components that have been sufficiently well-corrected for chromatic aberrations. Catadioptric systems that combine refracting and reflecting components, i.e., in particular, lenses and mirrors, are thus predominantly favored for configuring high-resolution projection lenses. Such systems frequently employ deflecting mirrors that are used at large angles of incidence and serve to deflect light traveling between the object plane and image plane of the projection lenses to one or more concave mirrors or to deflect light reflected by same back to same. In order that the mirrors will have high reflectivities, they will usually have reflective coatings, which will normally be multilayer reflective coatings. The curved surfaces, some of which may be sharply curved, of lenses will also usually be coated in order to reduce reflections, where light transiting some of same will also have large angles of incidence on same. However, employing dielectric coatings on optical components that involve large angles of incidence may affect transmitted light in various ways that will depend upon its polarization.
In the case of catadioptric systems of the aforementioned type, it has been found that the projected images of lines present on the patterns on the masks will frequently exhibit contrast variations that will depend upon the orientations of the lines. The variations in image contrast with orientation, which are also termed “horizontal-vertical variations” (H-V variations), will be reflected in discernible variations in the imaged widths of the lines on photoresists, where their imaged widths will depend upon their respective orientations.
Various means for avoiding such orientation-dependent contrast variations have been proposed. European Pat. No. EP 964 282 A2 concerns itself with the problem that catadioptric projection systems that employ deflecting mirrors introduce a preferred polarization direction for light transiting same that is attributable to their multilayer-coated deflecting mirrors having differing reflectivities for s-polarized and p-polarized light, which will cause light that is unpolarized at their reticle plane to become partially polarized when it reached their image plane, which, in turn, will cause their imaging characteristics to vary with orientation. According to the proposal presented there, this effect may be counteracted by creating a lead polarization by creating partially polarized light having a prescribed residual polarization within the illumination system involved that will then be compensated for by its projection lens such that light exiting the latter will be unpolarized.
A catadioptric projection lens having a polarization beamsplitter that is also sup-posed to minimize orientation-dependent contrast variations is known from European Pat. No. EP 0 602 923 B1, which corresponds to U.S. Pat. No. 5,715,084. The projection lens, which is used with linearly polarized light, has a means for altering the state of polarization of light transiting same that transforms incident linearly polarized light into circularly polarized light, which, in terms of the intensities of its orthogonally polarized field components, is equivalent to unpolarized light, situated between the projection lens' beamsplitter cube and its image plane, which is intended to provide that image contrast will be independent of the orientations of patterns appearing on masks. A corresponding proposal was also made under European Pat. No. EP 0 608 572, which corresponds to U.S. Pat. No. 5,537,260.
However, it has been found that contrast variations among patterns that have differing orientations on masks may still occur, particularly in the case of catadioptric projection lenses operated with large aperture and having at least one deflecting mirror, in spite of those measures to counteract same that have been described above.